Considerable research has been directed toward the synthesis of organic light-emitting diodes (OLEDs), in view their potential applications in full-color flat panel displays and solid state lighting. Such OLEDs often contain a light emissive layer comprising a luminescent material as a guest, dispersed and/or dissolved in a mixture of host/carrier materials capable of transporting holes, electrons, and/or excitons into contact with the luminescent guest. The luminescent guest is excited by the electrons, holes, and/or excitons, and then emits light. The light emissive layer is typically disposed between an anode and cathode. Single layer OLED devices are known, but typically exhibit very low quantum efficiencies, for a variety of reasons. Efficiency has been dramatically improved in some cases by employing additional layers of materials in the OLED devices, such as an additional layer comprising a material whose properties are optimized for transporting holes into contact with the emission layer, and/or an additional electron transport layer comprising a material whose properties are optimized for carrying electrons into contact with the emission layer. Upon application of voltage/current across the OLED devices, holes and electrons are transported through the intermediate layers and into the emissive layer, where they combine to form excitons and/or stimulate the formation of excited states of the luminescent guest material.
The luminescent guest materials can either be fluorescent materials that emit from a singlet excited state, or phosphorescent materials that emit light from a triplet excited state. While phosphorescent triplet emitters can potentially produce significantly enhanced quantum efficiencies as compared with singlet fluorescent emitters, the use of materials that emit from triplet states imposes additional requirements on the other materials of the OLED devices. In phosphorescent OLEDs, in order to reduce the excited state quenching often associated with relatively long exciton lifetimes and triplet-triplet annihilations, etc., the triplet guest emitters of the emission layers are typically inserted as guests into host materials. All the materials should be selected to optimize efficient injection of charges from the electrodes, in the form of holes, electrons, and the formation of singlet and triplet excitons, that are transferred as efficiently as possible by the host materials to the luminescent guest material.
In order to maximize energy transfer from the host materials to the guest phosphors, the energies of both the singlet and triplet states of the hole and/or electron carrying materials in the host should be higher than the energies of the corresponding singlet and triplet states of the guest phosphors. See FIG. 1. Furthermore, the conjugation length of the host materials should be limited, in order to provide a triplet energy level higher than that of the guest phosphors. Such triplet energy requirements become particularly challenging when designing host molecules that also provide the large charge (hole and/or electron) transport mobilities that are desired.
Thus, development of effective host materials for transporting holes, electrons, and excitons is as important as developing guest phosphors for the production of efficient OLEDs.
High-performance phosphorescent OLEDs with good short term luminescence and efficiency have been reported, but most such prior art devices have been fabricated by expensive multilayer vacuum thermal evaporation of small molecule electron or hole transport materials, to provide multi-layer OLED devices, as shown in FIG. 2. For example, host materials comprising carbazoles have been utilized as hole transporter and/or electron blocking materials in OLED applications. Examples of known small molecule carbazole-based hole-carrying materials are shown below. Polymeric carbazoles such as PVK are also known for use in the hole carrying layers of OLED devices.

Similarly, small-molecule 2,5-diaryl oxadiazoles such as those shown below (PBD and OXD-7) are known as suitable electron carrying materials for use in making electron carrying layers for OLED devices. Polymeric oxadiazole based electron transporting polymers have also been reported, such as for example PCT Application Serial No. PCT/EP/20008 068119 filed 19 Dec. 2008, claiming the priority of U.S. Provisional Application 61/015,777 filed 21 Dec. 2007, both of which are hereby incorporated herein by reference for their disclosures relating to monomeric oxadiazoles useful for preparing the disclosed polymers.

Furthermore, the use of “ambipolar” mixtures of hole carrying and electron carrying materials to form a mixed host material for phosphorescent guests in the emissions layers of multi-layer OLEDs are known. Nevertheless, devices based on mixtures of hole carrying and electron carrying materials in their emission layers, whether based on mixtures of small molecules and/or polymers tend to undergo phase separations, undesirable partial crystallizations, and/or otherwise degrade upon extended OLED device heating, decreasing OLED device efficiency and/or lifetimes over time.
Accordingly, there remains a need in the art for improved “ambipolar” host materials that can efficiently transport holes, electrons, and/or excitons into contact with phosphorescent guests in emission layers, without undergoing phase separations, crystallization, or thermal or chemical degradation. Furthermore, if a single “ambipolar” amorphous and polymeric host material could be used to transport holes, electrons, and/or excitons into contact with phosphorescent guests, it is possible that one or more of the electron carrying or hole carrying layers of the multi-layer OLED devices could be omitted, simplifying device design and manufacture, and lowering fabrication costs, especially if high cost vacuum deposition techniques could be replaced with lower cost solution processing techniques.
It is to that end that the various embodiments of the ambipolar polymers, copolymers, and materials and methods for their preparation described below are directed.